Polycystic kidney disease (PKD), the most common genetic cause of renal failure in humans, is characterized by the accumulation of fluid-filled cysts in the kidneys and other epithelial organs. Although the genes that cause PKD have been identified, the mechanism of cyst formation remains unclear. Recent studies suggest that PKD may arise from abnormalities of the primary cilium, an immotile, hair-like organelle that project from the apical surface of most renal epithelial cells. To test this hypothesis, Cre/loxP recombination was used to delete the Kif3a gene in the kidneys of transgenic mice. Kif3a (Kinesin family 3a) encodes the 80/85-kda subunit of the kinesin-II motor protein that is essential for cilia formation. Kidney-specific deletion of Kif3a results in viable offspring with normal-appearing kidneys at birth. Renal cysts begin to appear at postnatal day (P)5, and renal failure develops by P21. The cyst epithelial cells lack primary cilia and have abnormalities in cell proliferation, apoptosis, polarity, and (-catenin localization. The overall goal of this project is to understand how the loss of renal cilia leads to cyst formation. Inducible Cre/loxP recombination will be used to delete Kif3a in adult mice to test whether the loss of primary cilia in mature renal tubules produces PKD. Cyst epithelial cell lines established from Kif3a mutant mice will be examined for abnormalities in cilia formation, proliferation, and differentiation. The mechanism of increased proliferation will be elucidated, focusing on activation of the beta-catenin signaling pathway. The expression and subcellular localization of polycystin-1 and polycystin-2 will be examined, and the phenotype of mice with combined mutations of Kif3a and either Pkd1 or Pkd2 will be studied to determine whether Kif3a is in the same pathway of cyst formation as Pkd1 and Pkd2. Kif3a mutant cells that lack primary cilia will be exposed to fluid shear stress to elucidate the role of renal cilia in the flow-dependent regulation of intracellular calcium. Taken together, the characterization of a new mouse model of PKD promises to advance our understanding of the molecular pathogenesis of human cystic diseases and the essential role of primary cilia in regulating cell growth and differentiation.